The invention concerns a method in the drying of a moving web, such as a paper or board web, in particular in drying that takes place in connection with surface treatment or coating, in which the web is passed while being supported free of contact through various drying gaps in which the web is dried by means of infrared radiation and air blowings.
Furthermore, the invention concerns a combination dryer intended for carrying out the method herein, this combination dryer comprising an infra or infrared unit and an airborne web dryer unit integrated in its connection (i.e. connected thereto), the web being passed through the treatment gaps in these units while being supported free of contact by means of air blowings and while being dried by means of the infrared radiation and the air blowings. The combination dryer also comprises a circulation system for drying air and for the cooling air of the infrared dryer unit, and possibly also for combustion air. This circulation system comprises an outlet duct for circulation air and an inlet duct for circulation air, with blower equipment and possibly also heating equipment for circulation air, preferably a gas burner, being connected between these inlet and outlet ducts.
As is known in the prior art, paper webs are coated either by means of separate coating devices, by means of on-machine devices integrated in paper machines, or by means of surface-sizing devices which operate in a drying section of a paper machine so that the web to be coated is passed at the final end of a multi-cylinder dryer to a coating device, which is followed by an intermediate dryer and finally, e.g., by one group of drying cylinders as an after-dryer.
A typical application of the present invention is exactly in such an intermediate dryer situated after a coating device, to which however, the present invention is not to be confined.
In the prior art, so-called airborne web dryers are known in which a paper web, board web, or equivalent is dried free of contact. Airborne web dryers are used, e.g., in paper coating devices after a blade, roll or spread coater to support and to dry the web that is wet with the coating agent, free of contact.
In airborne web dryers, different blow nozzles for the drying and supporting air as well as arrangements of the same are applied. Such blow nozzles can be divided into two groups, i.e. positive-pressure or float nozzles and negative-pressure or foil nozzles.
The prior art airborne web dryers that are most commonly used are based exclusively on air blows. It is partly for this reason that the airborne web dryer becomes quite spacious or large-scaled, because the distance of effect of the airborne web dryer must be relatively long in order that a sufficiently high drying capacity could be obtained. A partial reason for these drawbacks is that in air drying, the energy density of the drying remains relatively low.
In the prior art, different dryers are also known which are based, e.g., on the effect of infrared radiation. The use of infrared radiation provides the advantage that the radiation has a relatively high energy density, which is increased when the wave length becomes shorter. The use of infrared dryers in the drying of a paper web has been hampered, e.g., by the risk of fire, because the temperatures in infrared radiators and in the environment become quite high in order that a drying radiation with a sufficiently short wave length can be achieved.
In a manner known in the prior art, in infrared dryers the thermal energy is supplied to the device either as electricity or as natural gas. The cost ratio between electricity and natural gas varies, so that the economy of use also varies. In electric infrared dryers, cooling air is required, whereas in gas infrared dryers combustion air is required which can, at the same time, for its part, also act as cooling air for the parts that become hot.
Due to the drawbacks described above, the starting point of the present invention has been an integrated combination of an infrared dryer and an airborne web dryer, in which most of the various advantages of these two dryers are carried into effect.
In the present invention, a gas-operated infrared unit is expressly intended to be used, because in such a case the advantages that are aimed at by way of the present invention are manifested best. In certain particular cases however, the present invention is also suitable, at least after certain modification, for application in conjunction with infrared units operating by means of electricity.
With respect to the prior art related to the present invention, reference is made to the previous Valmet FI Patent Applications Nos. 862427 and 872504. With respect to the prior art most closely related to the present invention, reference is made to the recent Valmet FI Patent Application No. 881603, entitled "Gas-Infrared Airborne-Web Dryer", by Sture Ahlskog.